The object of this research is to measure the detailed size- chemical nature of atmospheric particulates. This data is desired to evaluate the toxicity of atmospheric particulates which depends on both their inhalability (aerodynamic size) and their chemical composition. The data will be obtained in an aerosol beam device which (1) measures the aerodynamic size or mass of a particle by measuring its time of flight between two focused light beams in a vacuum chamber and (2) measures, by surface ionization mass spectrometry, the abundance of various charge to mass species present in individual particles. The measurements will therefore provide information about the chemical composition of the size subranges of atmospheric particulates. The data of particular interest concerns toxic species such as 1,2-benzopyrene, asbestos, and UO2, among others. If these data can be obtained in sufficient detail, it should be possible to identify the major sources of toxic atmospheric particulates. The instrument will be calibrated and the data will be collected and searched with the view of finding unique features of toxic and other aerosols which may serve to specifically identify their sources.